Course Objectives At the end of this course you should be able to: • • Identify the various functional groups that involve carbonyls Explain reaction mechanisms associated with each type of functional group

Nomenclature of Aldehydes and Ketones
• Common names are used for the simplest aldehydes and ketones:
O H C H

O C CH3 CH2 CH2 H

O C H

formaldehyde
O H3 C C CH3

butyraldehyde
O C CH3

benzaldehyde
O C

acetone

acetophenone

benzophenone

•

Common names are also used for carbonyl-containing substituent groups, which are known collectively as acyl groups:
O H C

O H3 C C

O C

formyl

acetyl

benzoyl
3

Nomenclature of Aldehydes and Ketones
• Traditional names are used for a great many aldehydes and ketones which were recognized as substances long before systems of nomenclature were developed:
H

O
O

H
H

O

C

H2 C

CH

C

O

cinnamaldehyde

furfural

acrolein

•

Three of the four bases which comprise DNA contain carbonyl groups (and all four bases are nitrogen heterocycles, which we will discuss later):
O N N H N
H3 C O
NH2

NH NH2
N H

NH O
N H

N O

guanine (G)

thymine (T)

cytosine (C)
4

Structure of Aldehydes and Ketones
• • • The carbonyl carbon of an aldehyde or ketone is sp2-hybridized. The bond angle is close to 120° (trigonal planar). The carbon-oxygen double bond consists of: – A ! C-O bond – A " C=O bond
We can compare the C=O bond length to those of C=C double bonds

5

Properties of Aldehydes and Ketones
• Aldehydes and ketones are polar molecules because the C=O bond has a dipole moment: For acetone: dipole moment = 2.7 D

Their polarity makes aldehydes and ketones have higher boiling points than alkenes of similar molecular weight. Aldehydes and ketones are not hydrogen bond donors (they can’t donate a proton); therefore, they have lower boiling points than alcohols of similar molecular weight. Aldehydes and ketones are hydrogen bond acceptors; this makes them have considerable solubilities in water.
O H O H R O C R' H H

•

Ketones such as acetone are good solvents because they dissolve both aqueous and organic compounds Recall that acetone is a polar, aprotic solvent.
6

Reactions of Aldehydes and Ketones
• The reactions of aldehydes and ketones can be divided into two main categories: – Reactions of the carbonyl group (Ch. 19) – Reactions involving the !-carbon (Ch. 22)

The polar effect of the oxygen in the carbon-oxygen bond attracts electrons. But… electron-attracting groups adjacent to carbocations are destabilizing However, the resonance stabilization outweighs this destabilization
12

Basicity of Aldehydes and Ketones
• Compare pKa values of the conjugate acids of aldehyde/ketones with those for the conjugate acids of alcohols:
+ O R H R' R H O + H

pKa: •

-7

-2.5

Protonated alcohols are less acidic; therefore alcohols are more basic than ketones

Does this make sense? – Resonance stabilization of the protonated ketone should make it less acidic (less likely to lose a proton) – Hydrogen bonding explains this apparent contradiction: protonated alcohols can undergo hydrogen bonding with two protons, protonated ketones only with one
+O R H R' OH2 R H OH2 OH2 O + H

14

Reversible Additions to Aldehydes and Ketones
• • Addition of water to an aldehyde or ketone gives a product called a hydrate or a gem-diol (two -OH groups on the same carbon). The reaction is both acid-catalyzed and base-catalyzed.
O + H2 O H3 C H H3 C OH C OH H

• •

The addition reaction is highly regioselective. – Addition always occurs with oxygen adding to the carbonyl carbon atom. The trigonal planar, sp2-hydridized carbonyl becomes tetrahedral, sp3hybridized in the addition reaction. The addition reaction is reversible. The equilibrium conversion to the hydrate varies widely and depends on the nature of the groups attached to the carbonyl group.
15

Addition Under Basic Conditions
• • Addition of water to carbonyl compounds under basic conditions has no analogy in reactions of alkenes The reaction occurs in two steps: – Addition of OH- to carbonyl carbon – Protonation of carbonyl oxygen • Addition occurs directly because OH- is a more reactive nucleophile than H2O

H

O H

OH

!

C

O

HO

C

O

!

HO

C

OH

+

OH

!

•

Note that regioselectivity of addition is the same for acid or base catalyzed nucleophilic addition

17

Geometry of Nucleophilic Attack on Carbonyl
• • The sp2 hybridization of the carbonyl compound means that attack of the nucleophile on the carbonyl carbon may occur from either face. The resulting addition product is sp3-hybridized.

Nu:$ •

Nu

We used the example of hydration (formation of gem-diols) to illustrate nucleophilic addition to carbonyl compounds; however, other nucleophiles can undergo the same reaction. There are other acid- and base-catalyzed examples.
18

•

Equilibria in Carbonyl-Addition Reactions
• • Carbonyl addition reactions are reversible. The extent to which the reaction is able to proceed is defined by the magnitude of the equilibrium constant:
OH

O + H-Nu: R R'

Keq

OH R C Nu R'

Keq =
O R

R

C Nu

R'

.
R'

H-Nu:

O

O H H3 C H H3 C

O CH3

Compound: Keq:

H

2000

1

0.001

This trend can be explained by looking at factors which affect the stability of the reactant and factors which affect the stability of the product.
19

H-Nu = H2O

Equilibria in Carbonyl Addition Reactions
• Reactant Stability: – Recall that alkyl groups stabilize double bonds (more highly substituted alkenes are more stable than less substituted alkenes) – This works for C=O double bonds, too – Ketones are more stable than aldehydes – Therefore, the addition to ketones is less favored than addition to aldehydes
Ketone: reactant more stable, product less stable

%G0
(ketone)

%G0 (aldehyde)

Aldehyde: reactant less stable, product more stable

•

Product Stability: – The four groups in the product are closer together than the three groups attached to the carbonyl carbon in the reactant. – Alkyl groups cause more steric destabilization in the tetrahedral addition product than does hydrogen – Therefore, the ketone addition product is less favored than the aldehyde addition product

Formation of Hemiacetals and Acetals
• When an alcohol adds reversibly to an aldehyde or ketone, the product is called a hemiacetal.
– Recall our example of the reaction between CH3OH and PhCHO.

•

Hemiacetals are formed in both acid- and base-catalyzed reactions.

O C + R OH

acid or base

OH C OR

Note: this bond to carbon indicates that it may be either to an alkyl group or to hydrogen.

hemiacetal

• •

Hemiacetals are unstable and can’t be isolated in most cases. Hemiacetals undergo further reversible reactions under acidic conditions only. – This reaction involves carbocation chemistry.
22

Formation of Acetals
• Hemiacetals react further with alcohols under acidic conditions to form acetals.

catalyzed by acid or base

catalyzed only by acid

O C + R OH

acid or base

OH C OR

R OH

OR C OR + H2 O

aldehyde or ketone

hemiacetal

acetal

23

Mechanism of Acetal Formation
• • • • • Under acidic conditions, some of the alcohol becomes protonated ROH2+. The hemiacetal OH oxygen abstracts a proton from ROH2+. Loss of water gives a resonance-stabilized alkoxy carbocation. Nucleophilic attack by the alcohol on the carbocation occurs. Deprotonation by a further alcohol molecule produces the acetal.
R H

H O C OR

H

+

O

+

H O C OR H

+ R O C

R O C +

+

H2 O

+ ROH
R O C + H O R
OR C O H + R

OR

H

O

R H

C OR

+

O

R H

24

Acetals as Protecting Groups
• • • The reversibility of acetal formation along with the relative inertness of the RO-C-OR linkage make acetals useful as protecting groups. Protecting groups are functional groups which may be introduced in a molecule by converting another functional group in a reversible reaction. If the protecting group is more inert than the original functional group, then other reactions may be carried out with this molecule without worrying about altering or destroying the protecting group. When the other desired reactions are completed, the original group may be restored by carrying out the reverse of the reaction which introduced the protecting group.
O C + ROH OR C OR OR C OR

•

H3 O+

O C

protection
(acetal formation)

further reactions

deprotection
(reverse of acetal formation)
25

Cyclic Hemiacetals
• Cyclic hemiacetals containing five and six atoms in the ring can form spontaneously from hydroxyaldehydes:
HO HO O H H O

alcohol and carbonyl functions are contained in the same molecule

Cyclic hemiacetals are more stable than noncyclic hemiacetals; they can be isolated. They are favored under equilibrium (large Keq) conditions. This oxygen becomes an OH group.
HC O

•

Five and six-carbon sugars are important biological examples of cyclic hemiacetals:

Reactions of Aldehydes and Ketones with Amines
• Aldehydes and ketones react with primary amines to form imines, or Schiff bases.
O H NH2 +

heat
N + H2O

An imine is a compound with a C=N double bond ( a nitrogen analog of an aldehyde or ketone

•

The mechanism of imine formation involves the nucleophilic addition of the amine to the carbonyl carbon, forming a stable intermediate species called a carbinolamine.
O H

Carbinolamines are compunds with an amine group and a hydroxy group attached to the same carbon.

C R H N

Carbinolamines are analogous to hemiacetals.

28

Mechanism of Carbinolamine Formation
• • • Carbinolamine formation begins with nucleophilic attack on the carbonyl carbon. The product of this attack is a neutral, charge-separated species. Water and its conjugate acid both play roles in the reaction.
!
H H O+ H

O

H N H R
R H

O

O

H + H2 O

N + H

R H

N + H

Protonated water as a Bronsted acid: nucleophilic attack of the negatively charged oxygen of the intermediate on protonated water.
O H

H

O H

Water as a Bronsted base: nucleophilic attack of the oxygen of water on a proton of the positively charged amine.

Enamines have a nitrogen bound to a carbon which is part of a C=C double bond.

•

The mechanism involves nucleophilic addition of the amine to the carbonyl to form a carbinolamine. Enamines form only if the carbonyl compound has at least one hydrogen on a carbon adjacent to the carbonyl carbon. Formation of the alkene may be recognized as an elimination reaction.
H

OH N O

carbinolamine

•

H OH N O

•

H

H

30

Irreversible Addition of Aldehydes and Ketones
• • • Aldehydes and ketones also undergo addition reactions which are essentially irreversible. These reactions use organometallic reagents to add alkyl groups to the carbonyl carbon atom. These reactions are nucleophilic additions; this means that the organic group added is acting as a nucleophile in the reaction.
We’re accustomed to electrophilic behavior of alkyl groups (I.e., carbocations), but how does an alkyl group act as a nucleophile?

R-Li R-MgX
(Grignard Reagent)

think of as….

R:! Li + R:! MgX +

The alkyl groups act as if they were free carbanions
31

Organometallic Reagents
• • The actual structure of Grignard reagents in solution is complex. These reagents are always prepared in ethereal solvents, and in fact two ether molecules are associated with the Grignard reagent.
O

Grignard reagent prepared in diethyl ether

R Mg X O

Grignard reagents are highly polar compounds and are very strong Lewis bases.

•

Mechanism of addition of a Grignard reagent to a carbonyl compound:
O C

Addition Using Organometallic Reagents
• Formation of alcohols via addition of Grignard reagents to aldehydes and ketones is carried out in two separate steps Step 1: Addition of the nucleophilic alkyl group to the carbonyl carbon, aided by Lewis acid interaction between MgX+ and the carbonyl oxygen. The product of this step is a halomagnesium alkoxide (see previous slide). Step 2. Protonation of the alkoxide oxygen. The product of this step is an alcohol.
This notation makes it clear that the overall reaction occurs in two separate steps:
H O C
2+ ! R + Mg X + H2 O

H O! C R
+

+

OH2

MgX

O H3 C C CH3 H 1. H3 C 2. H / H2 O + Mg
+

OH MgBr H3 C CH CH3
2+

CH3

halomagnesium alkoxide

Br! + H2O

Step 2.
33

Addition Using Grignard Reagents
• Primary, secondary and tertiary alcohols may be formed in the reactions of aldehydes or ketones with Grignard reagents.

Additions Using Organometallic Reagents
• • The net effect of the reaction of a Grignard reagent with an aldehyde or ketone in the addition of the components R and H across the C=O double bond. Compare to nucleophilic addition of HCN: Grignard: Nucleophilic addition of HCN
O C H CN

O C

1. R
+

MgBr

O C

H R

O C

H

CN

2. H / H2O

Nucleophile attacking the carbonyl carbon: A separate step is needed to produce the alcohol from the halomagnesium alkoxide irreversible reaction

“ R$ ”

Nucleophile attacking the carbonyl carbon:

CN$

“one-pot” reaction (multi-step mechanism, but all components may be added at once). reversible reaction
35

Addition Reactions Using Organometallic Reagents
• • • Addition reactions to aldehydes and ketones using Grignard reagents are among the most important reactions in organic synthesis. These reactions are carbon-carbon bond forming reactions. The net result of the reaction is the addition of the elements of R-H across the C=O double bond
O C 1. R MgBr 2. H / H2 O
+

O H C R

O

O E + E Y C Y

compared to..

C

•

Other metal-based reagents also add components across the C=O bond in an analogous way: – Metal hydrides add H- and H+ across the C=O double bond. – When we add H-H to a double bond, we call the reaction a reduction.
36

The hydride ion in LiAlH4 is more basic than the hydride ion in NaBH4, and therefore it is more reactive. Some functional groups which may be reduced by LiAlH4 are unreactive with NaBH4 (e.g., alkyl halides R-X, nitro groups -NO2) – Therefore NaBH4 may be used to reduce C=O bonds in the presence of such groups.
37

reactions at the carbonyl group reactions at the carboxylate oxygen reactions involving #-hydrogens
38

H
Bronsted acidity

Lewis acidity

Nomenclature of Carboxylic Acids
• • Many carboxylic acids are known by their common names. These common names are often formed by adding the suffix “ic” + acid to the common names for aldehydes and ketones.
O O H OH H3 C O OH OH

Structure and Properties of Carboxylic Acids
• Compare bond lengths and angles to other C=O and C-O containing compounds:
sp2-sp3 single bond is shorter than sp3-sp3 bond C=O bond length is the same in aldehydes, ketones, and carboxylic acids

O R O

H H

O O

R

•

Carboxylic acids have high boiling points due to their ability to act as both donors and acceptors in hydrogen bonding.

Separation of charge in the carbonyl C and O is already partially developed in carboxylic acid

41

Basicity of Carboxylic Acids
• • As in aldehydes and ketones, the carbonyl oxygen of carboxylic acids is weakly basic. The carbonyl carbon of carboxylic acids reacts with protons to form a resonance-stabilized conjugate acid.
O + H3 O R OH

+

O R

+

H OH

O

H

O R

H
+ H2 O

R + OH

OH +

•

Why does protonation occur only at the carbonyl oxygen, and not at the carboxylate oxygen?
O R + O H H

• •

Protonation at the carboxylate oxygen would result in a species like this This species is not resonance-stabilized, so its formation is much less favorable
42

Reactions at the Carbonyl Group
• The most typical reaction at the carbonyl group of carboxylic acid is substitution at the carbonyl carbon: Another important reaction is the reaction of the carbonyl oxygen with an electrophile: Reaction at the carboxylate oxygen results in ionization:
O R OH
O R OH O + E Y R Y + E OH

•

+ + E Y R O E OH + Y ! R

O

E + OH

•

O R OH

+ H2 O R

O O !

!

O O

R + H3 O +

•

The carboxylate oxygen anion can act as a nucleophile:

O + R O ! E Y R

O O E

+

Y

!

43

Conversion of Carboxylic Acids into Esters
• The acid-catalyzed preparation of esters from carboxylic acids is known as Fischer esterification. – Esters are carboxylic acid derivatives with the carboxylate -OH group replaced by an alkoxy group. – Treating a carboxylic acid with an excess of an alcohol gives esters.
O C OH + H3CO H H2 SO4 O C OCH3 + H2O

excess

• • •

This is an example of substitution at the carbonyl carbon. The reaction is driven forward by using a large excess of alcohol (usually as solvent), an application of Le Chatelier’s priniciple. This esterification can’t be carried out with tertiary alcohols (why?) or with phenols.

44

Mechanism of Acid-Catalyzed Esterification
• The first steps of the reaction are familiar to us from aldehyde/ketone nucleophilic addition: – Protonation of the carbonyl oxygen by CH3OH2+. – Nucleophilic attack of the alcohol on the carbonyl carbon by CH3OH. – Deprotonation of the tetrahedral intermediate by CH3OH.
+ H3CO O R OH H H

+ H O + R OH H3CO H
R

OH + OH
R

OH OH

CH3 O

CH3 O +
H

protonation

addition

H
H3 CO

H3 CO

H H

•

This reaction differs from addition to aldehydes and ketones in the next step -- in what happens to the tetrahedral intermediate.

deprotonation

45

Mechanism of Acid-Catalyzed Esterification
• In acid-catalyzed reactions of aldehydes and ketones, the tetrahedral alcohol species formed in these three steps (protonation, addition, deprotonation) is a hemiacetal. It may undergo further reactions, but the ultimate product will be a tetrahedral addition product. In acid-catalyzed reactions of carboxylic acid, this tetrahedral species is not stable and undergoes further reaction, ultimately producing a substitution product and not an addition product.
OH R CH3 O OH H H + OCH3

In both reactions, a tetrahedral product is initially formed. The choice between substitution and addition depends on how good the leaving group, X, is.
O R X E Y

For aldehydes and ketones: X = R, H Cannot act as leaving group For carboxylic acids: X = OH, protonated to OH2+ Can act as leaving group
47

tetrahedral (intermediate) product

Esterification by Alkylation
• Another route to esterification of carboxylic acids involves one of the other types of reactions we outlined at the beginning of the discussion of carboxylic acids. reactivity at the carboxylate oxygen • Alkylation of the carboxylate oxygen may be carried out using diazomethane, a toxic, explosive, allergenic gas.
O + R OH ! H2C + N O N R OR + N2

This reaction fits the pattern of substitution at the carbonyl group; however, the mechanism proceeds slightly differently from that of Fischer esterification. The first step involves attack of the sulfur of thionyl chloride by the carbonyl oxygen acting as a Lewis base.
O

O R O H Cl

O S
O

S

Cl Cl H

!

The sulfur is very electrophilic; why?

Cl
R + O

49

Preparation of Acid Chlorides
Compare reaction of carboxylic acids with SOCl2 to reaction with alcohols. • • Carbonyl oxygen acts as a Lewis base to attack the electrophilic sulfur atom instead of attacking a proton. The protonated product of this first step is a very powerful electrophile. – The Cl- anion can abstract a hydrogen, giving an unstable intermediate… – Or, the Cl- anion can undergo nucleophilic attack on the carbonyl carbon (because this species is so electrophilic, even a weak nucleophile like Clcan undergo this reaction).
O O R + S Cl H ! Cl R O O S Cl R O + O S Cl H ! Cl

O O R S Cl

O

O HCl

O

Cl

OH

50

Preparation of Acid Chlorides
• The last step is the elimination of the thionyl chloride group as SO2 and HCl. This step is irreversible because SO2 is a gas.
H O R O O S Cl
R O + HCl +SO2 Cl

Cl

Redraw two of the intermediates in this mechanism: Drawn this way, it makes it look like a simple substitution at the carboxylate oxygen. However, if we look back at the mechanism, we can see that the thionyl chloride adds to oxygen in a carbonyl group, not to oxygen in a carboxylate group. The reaction occurs this way because the carbonyl oxygen is more strongly Lewis basic than is the carboxylate oxygen.
R
O R

O S Cl

O O R S Cl

O

Cl

OH

redraw
O O O S Cl O R OH

OH R O

O S Cl

Cl

51

Attack via the Carbonyl Oxygen
• The oxygens in a carboxylic acid are indistinguishable. The proton moves rapidly back and forth from one to the other.
O R O H R O H O

….however, when the reaction takes place, it does so via attack by the more Lewis basic carbonyl oxygen (whichever one that happens to be at that instant in time), and not via the less basic carboxylate oxygen.
O R O O S Cl

The oxygen that is a carbonyl in this intermediate was a carboxylate when the reaction started.
52

Preparation of Acid Chlorides
• Another interesting point about the mechanism of acid chloride formation is the role of the thionyl intermediate. protonated intermediate (not isolated)
O O S Cl H R O

isolable intermediate
O S Cl

reactants
O R OH + Cl O S Cl

R

+

O

O

This species is present in only a very small concentration because it is so unstable. The isolable intermediate is not a “true” intermediate species, since it is not on a direct pathway to the product.
R

This reaction is driven towards the isolable intermediate.
O Cl

product

The isolable intermediate serves as a kind of “reservoir” to supply the reaction pathway with the protonated intermediate.
53

Can you recall an analogous example?

Reactive Intermediates Have Low Concentrations
• • • If the minor species is too reactive, a high concentration might make it subject to unwanted side reactions. The concentration of the minor species is kept low by virtue of its reversible reaction to form the more stable major species. This “reservoir” meters the concentration of the minor species so that it reacts immediately to products and can’t build up in concentration.

Carboxylic Acid Derivatives (Acyl Compounds)
• • Not only are the structures of these compounds related, but their chemistries are related also. One of the most important types of reactions these compounds undergo are nucleophilic addition-elimination reactions resulting in the replacement of L with another nucleophile.
O

O

!
R

O + Nu L !

R Nu !

L

R Nu

L

addition of Nu:$

elimination of L:$

•

The two most important factors governing the chemistry of these transformations are: – The stability of the starting carboxylic acid derivative – The characteristics of the leaving group L:
$

Acyl chlorides are the least resonance-stabilized of the acyl compounds Halides are the best leaving groups of the acyl compound “L” groups
59

Structure and Stability of Acyl Compounds
• Let’s take a look at one of the high energy resonance forms and compare them for the different acyl compounds.
O ! O + NH2 ! O + OR ! + O O O ! + Cl

R

R

R

&+

R'

R

&+

N is less electronegative than O or Cl, so amides can bear the positive charge better than other L groups. The result is a more polar C=O group and a more basic carbonyl oxygen.

Oxygen is more electronegative than nitrogen and likes bearing the positive charge less. Anhydrides exhibit charge repulsion between the carbon in the leaving acyl group and the carboxylate oxygen

The strong polar effect of chlorine destabilizes the compound via an unfavorable carbonchlorine interaction - the most unfavorable in this series.

increased resonance stabilization

O

O

!

O

!

60

Reactivity in Nucleophilic Acyl Substitution
• The relative reactivity of acyl compounds in nucleophilic acyl substitution is affected by both of the factors we have discussed: – Stability of the starting acyl compound – Ability of the “L” group to act as a leaving group • We can use activation energy to show this more clearly.
!
TS R 2 Nu

O + Nu R L TS 1 R

O

O + L !

L Nu

!G1
The height of the first transition state will depend on the stability of the starting acyl compound

!G2
The height of the second transition state will depend on the proficiency of L as a leaving group

61

Reactivity in Nucleophilic Acyl Substitution

Case (a): transition states for formation and breakdown of tetrahedral intermediate are similar in energy (Nuc:- and L:- are similar).

Case (b): reactant is more stable relative to transition state 1 and L is a worse leaving group compared to Case (a).

Case (c): reactant is destabilized compared to Case (a), and L:- is a better leaving group in this case.
62

Reactivity in Nucleophilic Acyl Substitution
• The trends for reactant stability and leaving group ability tend to work together (both contributing to make the compound more reactive or both contributing to make the compound less reactive)
O O O O O

R

NH2

R

OR'

R

O

R'

R

Cl

INCREASING REACTIVITY

decreased resonance stabilization

weaker Lewis base - better leaving group

63

Reactivity of Acid Chlorides
• Acid chlorides are the most reactive of the acyl substituted carboxylic acid derivatives, and they readily undergo a variety of nucleophilic additionelimination reactions.
O

Notice that two moles of the amine are required to complete the reaction: one to act as the nucleophile which substitutes for Cl, and one to act as a base to deprotonate the ammonium ion.

•

Other methods for carrying out the proton transfer step have been developed for cases where the amine is too costly to waste in the final proton transfer step. – Add a tertiary amine to carry out the final proton transfer step (why tertiary?) – Run a two-phase reaction with a strong base (OH-) preent in the aqueous layer.
65

Formation of Amides From Acid Chlorides
• Tertiary amines such as pyridine or triethylamine can carry out the proton transfer step:
O
O

Reaction of Acid Chlorides
• Esters are formed from the nucleophilic substitution of acid chlorides with alcohols. – Hydrochloric acid is a by-product of this reaction. In practice, tertiary amines are often added to neutralize the HCl.
N O H3C H3 C H3 C OH + Cl O

Reactions of Anhydrides and Esters
• Anhydrides react with nucelophiles in substitution-elimination reactions in a manner similar to acid chlorides. amines anhydrides alcohols esters amides

•

Esters react with nucelophiles in substitution-elimination reactions in a manner similar to acid chlorides, although they are much less reactive towards these nucleophiles. amines esters alcohols esters amides
When an ester reacts with an alcohol to give another ester, the reaction is known as a transesterification.
68

Hydrolysis Reactions
• Carboxylic acid derivatives undergo a cleavage reaction with water (hydrolysis) to yield carboxylic acids. – When the reaction is acid-catalyzed, the mechanism is the reverse of the Fischer esterification of carboxylic acids. – When the reaction occurs under basic conditions, the base is not a catalyst but is consumed in the reaction. – Under basic conditions, the reaction is effectively irreversible.
O OCH3 + NO2 O O OH NO2 ! + CH3 OH

We can’t regenerate OH-, so the reaction is not catalytic in OH-, but instead consumes OH- as a reactant.
70

The reaction is strongly driven toward the right.

Hydrolysis Reactions
• Hydrolysis of amides can also be carried out under acidic (acid-catalyzed) or basic conditions, but the reaction is slower and must be heated to make it proceed. – Why it this reaction slower than that of acid chlorides or esters?

O H H3 C N CH3 + ! OH CH3 OH H2 O H3 C

O O! + CH3 NH2

•

Compare the leaving group in this reaction (-NHR) with that is an ester hydrolysis (-OR) (-NHR) is a much stronger base (its conjugate acid is much weaker) and therefore it’s a poor leaving group

Leaving group -OR -NHR

conjugate acid (pKa) ROH NH2R 15 35

•

71

Chemistry of Nitriles
• Nitriles are compounds of the same “oxidation level” as carboxylic acids and carboxylic acid derivatives (three bonds to heteroatoms).
C N

Note that the reaction stoichiometry under acidic conditions requires two moles of water for each mole of nitrile converted.

+ NH3

72

Hydrolysis of Nitriles
• Hydrolysis of nitriles under both acidic and basic conditions involves the formation of an amide, which is then hydrolyzed to a carboxylic acid as we have already discussed. Nitriles behave mechanistically like carbonyls. – Acid-catalyzed hydrolysis begins with protonation of the nitrile nitrogen, followed by addition and a series of protonation-deprotonation steps to form the amide. – Hydrolysis under basic conditions involves attack of OH- on the nitrile carbon as the first step.

•

73

Reduction of Carboxylic Acid and Acid Derivatives
• • Metal hydrides react with carboxylic acids to produce alcohols. The first step is different from the reaction between LiAlH4 and aldehydes or ketones: – the basic hydride abstracts a proton to create a carboxylate ion and H2(g).
Li O R C O H + H ! AlH2

O R C O ! Li +

+ AlH3 + H2 (g)

LiAlH4 R

O C + H

+ ! Li OAlH2

• •

A carboxylate salt is not a good electrophile! However, the LiAlH4 is able to undergo nucleophilic attack and reduce the carbonyl to an aldehdye (the mechanism still under debate, so we won’t go into the details of this step). Aldehydes react quickly with remaining LiAlH4 to produce alcohols as we have already discussed.
74

•

Reduction of Carboxylic Acid and Acid Derivatives
• LiAlH4 reduces acid chlorides to alcohols in a manner similar to the mechanism for aldehyde reduction. – The acid chloride is so reactive, the hydride ion can attack the carbonyl carbon directly without the carbonyl oxygen-Li+ interaction
Li O R C H H +

O
! AlH2

!

Li + Cl

R H

O R C H Cl

Li

+ ! AlH2

aldehydes and ketones

acid chlorides

• •

The acid chloride is reduced to an aldehdye which is then reduced to an alcohol. Reduction from aldehyde to alcohol can be prevented by using a less reactive hydride reagent:
Li + H ! Al O CH3 CH3 CH3 3

75

Chemistry at the #-Carbon of Carbonyl Compounds
• • • We have learned that the oxygen of a carbonyl group can act as a Bronsted or a Lewis base by attacking protons or other electrophiles. We have shown how the carbon of a carbonyl group can act as a Lewis acid resulting in addition of a nucleophile. Now we will explore chemistry at the carbon atom adjacent to the carbonyl carbon (#-carbon).
Lewis base chemistry at # -hydrogen

•

What makes the #-carbon special? – Hydrogens attached to the #carbon of aldehydes and ketones are weakly acidic. – How acidic are they?

O H C
R2

R1
#-carbon

C

R
Lewis acid

pKa values range from ca. 15-20 (similar to alcohols).

76

Bronsted Acidity of Aldehydes and Ketones
• Ionization of a proton from the #-carbon of an aldehyde or a ketone via attack by a Bronsted base results in formation of an anion called an enolate.

B

! H C

O C R

O ! C C R

O C C

! R

pKa = 16 •

+

B

H

Compare to the ionization of an proton further removed from the C=O bond:
not acidic

H H H C H H C

O H C C H R
#-hydrogen acidic
77

Hydrogens on carbon atoms not adjacent to the carbonyl carbon can’t form resonance-stabilized conjugate bases

Bronsted Acidity of Aldehydes and Ketones
• Ionization of a proton from the #-carbon of an aldehyde or a ketone via attack by a Bronsted base results in formation of a resonance-stabilized anion called an enolate.
B ! H C O C R
O ! C C R

Acidity of #-Hydrogens
• • Enolate anions are resonance-stabilized. The extra stability afforded the conjugate base by resonance stabilization makes the #-hydrogen of an aldehyde or a ketone more acidic than an #hydrogen on a compound which can’t form resonance-stabilized intermediates. But this can’t be the whole picture…..
!

In the highest energy occupied orbital, the oxygen has more of the negative charge but the carbon has more of the orbital

!

This higher energy orbital is where the electrons come from when the enolate ion acts as a Lewis base.

The lower energy orbital shows orbital overlap over the entire ion.

There are three M.O.’s of the enolate ion, derived from the three 2p orbitals involved in the overlap; the third (not shown) is an unoccupied antibonding orbital.

81

Reactions of Enolate Ions
• • How do we know whether the attack of the enolate ion will occur at the #carbon or at oxygen? The molecular orbital picture tells us that: – The enolate electrons which will participate in a nucleophilic attack reside predominantly on the oxygen. – The orbital which participates in the nucleophilic attack is distorted toward the # -carbon. Reactions which are dominated by charges and electrostatic interactions occur at the oxygen. Reactions which are dominated by orbital interactions occur at the #-carbon.

• •

82

Reactions of Enolate Ions
• • Remember that resonance structures are not separate entities, but that the molecule is some weighted average of these structures. We can write the curved arrow mechanism using either resonance structure. – Example: attack of the enolate carbanion on a carbonyl carbon:

O C R

! C

O + C
R

O
C C

O C

!

!

O C R C
O + C

O
C R C

O C

!

83

Bronsted Basicity of Enolate Ions
• When the enolate ion acts as a Bronsted base, it may use either the #-carbon or the oxygen to attack a proton. – When enolate ions attack protons, either a C-H bond or an O-H bond may be formed. – If a C-H bond is formed, the original aldehyde or ketone is regenerated. – If a O-H bond is formed, the product is an isomer of the aldehyde or ketone known as an enol.
O R C H C + B!
B H
B

H

O C C + B!

R
H

aldehyde or ketone

O C R

! ! C

O C R C

enol

84

Formation of Enols
• The conversion of a carbonyl compound into its enol is an isomerization reaction called enolization.
H R O C C

O R C

H C

ketone

enol

• • •

For most simple aldehydes and ketones, the equilibrium concentration lies far on the side of the ketone. The reaction is catalyzed by both acids and bases. The mechanisms for acid- and base-catalyzed enolization are different.

Simple aldehydes and ketones have very small equilibrium constants for enol formation.

Keq" 10-20
OC2 H5 H2 C

OH

Esters are even less favored to form enols.
OC2 H5

ethyl acetate
O H H3 C H CH3 H3 C H CH3 O

Keq" 10

H O O

'-dicarbonyl compounds form relatively stable enols due to resonance stabilized conjugation and the opportunity for hydrogen bonding.

O H H

OH

Keq" 1014
phenol

Phenol is the “ultimate enol”. Its aromaticity makes it significantly more stable than its keto form.
86

Mechanisms of Enolization
• The enolate ion is the intermediate species in base-catalyzed enolization of aldehydes and ketones. !
+ O H H

+
O C R C

O

H

O R C

H C +

! O

O
H

C R

! C

!

H R

O C C

ketone •

enolate ion

enol

A protonated keto/enol is the intermediate species in acid-catalyzed enolization of aldehdydes and ketones.
+
+ H O

O H H

+

O H

+ H

H
+

H O
+

H

O R C

H C +

H H

O C R

H C

H C

H R

O C C

C

R

ketone

protonated keto/enol

enol

87

Implications of Enol Formation
• In base-catalyzed enol formation, enolate ions are resonance-stabilized species in which the C-C-O linkage becomes trigonal planar and sp2-hybridized.
– This means that any chiral information contained in the aldehyde or ketone is lost upon formation of the enolate ion.

•

In acid-catalyzed enol formation, the #-hydrogen is removed to form a trigonal planar sp2-hybridized C=C bond.
– Chiral information in the aldehyde or ketone is lost in formation of the enol.
O C C Ph O C Ph C Ph H CH3

starting compound
O C Ph C Ph H

O
! OH

C H

! CH3 C Ph H O

!

O C C

CH 3 Ph

both enantiomers Ph are formed:

Ph H

CH3

H
+
+

H H C C CH3 Ph O H
+

chiral information is lost here
H C CH3 Ph

CH3 H

O

O C

H H

H O C Ph C

CH3
+

H

O

H H

Ph

Ph H

Ph

H

O

88

Aldol Addition and Aldol Condensation Reactions
• Aldehydes and ketones undergo a reaction called the aldol addition to form "hydroxy aldehydes and ketones:
O
+

O H H3 C H

OHH2 O H3 C

OH CH2

O H

H3 C

• •

Under more severe basic conditions, or under acidic conditions, the reaction proceeds further. Dehydration to form an #,'-unsaturated carbonyl compound is called the aldol condensation:
O
+

O H H3 C H

concentrated base heat H3 C

OH CH2

O H
H3 C

O + H2 O H

H3 C

•

These reactions are addition-elimination reactions which proceed via acid and base-mediated mechanisms which should be familiar to us by now.
89

Acid-Catalyzed Aldol Condensation
• The '-hydroxy aldehyde is deprotonated to form the aldol addition product:

+

O O H CH2 OH H H CH3 H
H O CH2 OH H CH3 + H3 O+

H

•

Under acidic conditions, the aldol addition product is not stable; it undergoes acid-catalyzed dehydration to form the aldol condensation product:
O + H2 O H CH3

O H CH2

OH H CH3

H 3 O+

(write the mechanism of this dehydration)
92

Base-Catalyzed Aldol Condensation
• • • Aldol condensation also occurs under basic conditions. Dehydration is more difficult in base because OH- is a poor leaving group. More concentrated base or heat helps to drive the reaction.

O H H C H

OH H CH3

OH!
H

O CH
!

OH H CH3

H2 O

O H CH
!

OH H CH3
H

O
+

!

OH

CH3

• •

Simple alcohols do not dehydrate under basic conditions. Why does this reaction proceed for '-hydroxy aldehydes and ketones?
93

Crossed Aldol Additions
• What happens if an aldol addition reaction is carried out with two different carbonyl compounds?
O
+

O CH3 H C H2 CH3

H

O H

OH H CH3 H

O

OH H C2 H5
H

O

OH H CH3 CH3 H

O

OH H C2 H5 CH3

•

Four different aldol addition products are possible!

94

Claisen-Schmidt Condensation
• How can we limit the number of combinations possible in a base-catalyzed aldol condensation reaction? – Think about the intermediate species
O H H C H H OH! H O
!

+ H2 O

CH2

We can’t form an enolate unless we have #-hydrogens

•

If only one of the two carbonyl compounds has #-hydrogens, only one can form an enolate ion. – So now we’re down to a maximum of two possible products. – Can we be even more selective? What reaction product(s) do we expect from this reaction?
O
+

can’t form an enolate forms an enolate which reacts much faster with its aldehyde partner than with itself

condensation cross product

•

Remember that ketones are generally more stable than aldehydes. – This means that ketones generally have a greater “energy hill” to climb to reach the transition state, and the rate is slower than for aldehydes.

•

Addition to an aldehyde is also generally more favorable thermodynamically. – This means that the balance of products at equilibrium lies toward the cross product shown above.
96

Intramolecular Aldol Condensation
• If a molecule has two carbonyl functions, there is a possibility that intramolecular aldol condensation may occur.

O CH3 O acid catalyst O

= site of enolate carbanion

•

Intramolecular aldol condensation is favorable when five- or six-membered rings may be formed.

97

Synthesis with the Aldol Condensation
• With what we know about the mechanism of the aldol condensation, we should be able to deconvolute the origins of any aldol condensation product. – Identify the carbonyl compound which was the enolate or enol compound. – Identify the carbonyl compound which was attacked by the enolate or enol. – Then ask the question: is this a feasible reaction? How complex will the product mixture be?
O R4
This portion is attacked by the enolate or enol

R1 R3 R2

This portion forms the enolate or enol

O R4 R3
R1

O CH2 R2
98

Synthesis With the Aldol Condensation
• We can also identify the '-hydroxy aldehyde or ketone which was formed from our starting materials as the first step before the aldol condensation.
O R4
This portion is attacked by the enolate or enol

Claisen Condensation
• An ester enolate ion attacks an ester to form a '-keto ester:
!

O Et H3 C O ! H2 C

O Et O
H3 C

O C OEt

O C C H2 O

tetrahedral Et intermediate

ester pKa = 25
O H3 C ! CH O Et O

ester enolate

EtO
+ EtOH

!

O H3 C C H2

O Et O EtO !

"-keto ester
We can drive the reaction to completion by using one equivalent of the ethoxide base instead of a catalytic amount pKa = 10.7

"-keto esters are much less stable than the ester starting material.
101

Claisen Condensation
• • We can use the acidic hydrogen adjacent to the two carbonyl groups to form the salt. We acidify the solution afterwards to regenerate the unionized '-keto ester.

O H3 C ! CH

O Et O

H3 O+ H3 C

O C H2

O Et O H2 O

This means that we need to have at least two #-hydrogens on our ester; • one to form the original enolate ion to form the C-C bond. • one to form the ionized '-keto ester product.
(the starting ester in this example actually had three !-hydrogens)

102

Claisen Condensation: Summary
• We need to have at least two #-hydrogens on our ester; – – one to form the original enolate ion to form the C-C bond. one to form the ionized '-keto ester product.
formation of '-keto ester is unfavored
O Et H3 C O ! H2 C O Et O
H3 C C H2 O O O Et

There is also a crossed Claisen condensation, which works best if one ester has no #-hydrogens or if one ester is especially reactive.
O O Et O O + H O

O
O Et Na EtOH AcOH

O H O O O Et
104

Et

Et

(two separate steps)

Summary of the Claisen Condensation
• The Claisen condensation consists of the following five steps: 1 Formation of an ester enolate anion 2 Addition of the enolate to the ester (carbon-carbon bond forming step) 3 Elimination of alkoxide from the tetrahedral intermediate species 4 Removal of #-hydrogen from the '-keto ester (to form another enolate) 5 Acidification (in a separate reaction) to regenerate the '-keto ester • Base is a reactant, not a catalyst in the Claisen condensation: – one full equivalent of base is required in order to form the the ionized product in step 4 above. • The base used should be the same alkoxide (-OR-) as is present in the ester. (why??)
O H
EtO !

Et C H2 O

105

Synthesis With the Claisen Condensation
• A Claisen condensation product may be thought of as the result of adding the elements of an alcohol R-OH across the carbon-carbon bond between the two carbonyl groups.

O 2 H3 C O Et EtONa EtOH H 3 O+ H3 C

O H C H

O Et O
+ EtO

H

EtO ( H
O Et H3 C O

O Et H C H2 O

acceptor carbonyl compound

enolate component
106

Synthesis With the Claisen Condensation
• There are always two different ways to break up (mentally) a Claisen condensation product – this time we’ll break it up on the other side of the carbon in between the two carbonyl groups):
O H Et H3 C C H O O

H ( OEt
O

O

H
H3 C CH2

Et

Et

O

O

enolate component

acceptor carbonyl compound
107

Synthesis With the Claisen Condensation
• After we determine the potential starting materials for a Claisen condensation, we can then examine them and ask whether the reaction will be practical. – Will the reaction give the desired product in good yield or will there be a mixture of products? – Are the starting materials inexpensive and easily obtained?
O R1 CH R2 R3 C H O O Et

Work backwards from the product

1

2
R3

1
O
R1

2
O

enolate component: acceptor carbonyl compound:

Et H2 C
O R1 CH R2

O

CH R2

C H2
O

R3

O

Et

Et

Et

O

O

108

Alkylation of Enolate Ions
• Esters may be converted completely to enolate ions using very strong, branched bases like lithium diisopropylamide (LDA):

!

N

Li

+

O H Et H2 C O
!

O NH Et O H2 C

LDA

pKa ! 25

The reaction is driven to the right

pKa ! 35

•

The enolate ion that is formed may be used in a subsequent (separate) step as a nucleophile to attack alkyl halides in an SN2 reaction.
O
!

The bulky alkyl groups on LDA experience severe van der Waals repulsions with groups on the carbonyl compound, retarding the rate of reaction at the carbonyl C compared to reaction at the #-hydrogen.

•

Why doesn’t the enolate ion attack its own ester?
! N Li +

O Et H3 C O

O

O
!
H3C O

Et

O Et O C H2

O Et O

Et O

H2 C

slow

much faster reaction The ester is totally consumed before (enolate+ester) have time to react together

110

Enolate Formation and Reactivity
• When bases such as ethoxide attack esters to form ester enolate ions, the reaction is never complete (it’s driven to the left).
– – This means that the solution will consist of a mixture of the starting ester and a small fraction of its enolate anion. This allows the Claisen condensation to take place.
O H C H2 O Et

EtO! H2 C

O Et O
+ EtOH

ester and enolate both exist in solution

•

When much stronger bases such as LDA are used, enolate formation is rapid and complete (it’s driven to the right).
– This means that the solution will consist solely of the enolate anion. – This allows us to carry out alkylation without Claisen condensation as a side reaction.
! N Li +

The enolate may be alkylated with alkyl halides as we have just shown. A second alkylation may also be carried out using a different alkyl halide (in a separate step!).
– This provides flexibility in synthesis, creating two C-C bonds in the same molecule.
O O Et O C H2 O
O Et R1 O C R2 O O Et

Acetoacetic Ester Synthesis
• Ketones may be formed from '-keto esters like ethyl acetoacetate via a series of reactions that proceeds via formation of an enolate ester anion.
B:RX

'-keto ester
O H 3C CH2 O Et O

enolate
O
!

alkylated '-keto ester
O O Et H 3C CH R O

O Et O CH

H 3C

ketone
O H 3C CH2 R

OH-/H2O

(saponification)

decarboxylation

H3O+/H2O (protonation)
H3 C

O CH R

O O
!

114

Conjugate Addition Reactions
• We learned that '-hydroxy aldehydes and ketones undergo dehydration to form #,'unsaturated carbonyl compounds.
– These compounds undergo further reactions at the C=C bond which are not found in the reactions of simple alkene compounds.

•

Nucleophilic addition to the C=C bond can occur: why?
– – A resonance-stabilized enolate intermediate is formed. The overall observed reaction is the net addition to the double bond.

H
O R1 R2
+ Nu

O R2
!

+

H Nu

!

O R2

R1

R1

Nu

H
Nu R1 CH H O R2

H Nu R1 O R2
115

Conjugate Addition vs. Carbonyl Group Reactions
• • • We know that nucleophiles can attack the Lewis acidic carbonyl carbon atom. Thus it seems reasonable that the conjugate addition reaction we’ve just described may compete with nucleophilic attack at the carbonyl carbon. How can we tell which reaction will dominate?

O

attack at C=O carbon:
R2

attack at C=C carbon: conjugate addition
R1

carbonyl addition, R2= R or H carbonyl substitution, R2= OR

Nu

H

•

We must consider both kinetics and thermodynamics to answer this question: – Kinetics: which reaction is faster? – Thermodynamics: which reaction gives a more stable product?
116

Competition in Reactions of Aldehydes and Ketones
• Kinetics: which reaction proceeds faster?
– Often the addition to the carbonyl carbon proceeds faster (because the carbonyl carbon is a stronger Lewis acid than the C=C carbon) – Therefore the carbonyl addition reaction usually proceeds faster.

In reversible reactions, the kinetically favored product eventually “drains” back through to the more stable thermodynamically favored product. When carbonyl additions are reversible, conjugate addition wins out.

The carbonyl addition product dominates with more powerful nucleophiles like concentrated OH-.

Nu

O R2

slower and irreversible

R1

119

Competition in Reactions of #,'-Unsaturated Esters
• Kinetics vs. Thermodyamics in nucleophilic attack on #,'-unsaturated esters: – With esters, when nucleophilic attack of a strong base occurs at the carbonyl carbon, the result is an acyl substitution instead of addition to the carbonyl carbon. – In contrast to aldehydes and ketones, these reactions at the carbonyl carbon are not reversible (think of saponification). • Acyl substitution products dominate in the nucleophilic attack of esters with strong bases.

Michael Additions
• • When the nucleophile in a conjugate addition to an #,'-unsaturated carbonyl compound is an ester enolate anion, this reaction is called a Michael addition. The first step of the addition reaction results in the formation of another enolate ion with the carbon alpha to the carbonyl group in the #,'-unsaturated carbonyl compound
O R3 O C H2 O Na OEt OR3
+ !

O
!

O C H OR3

R3 O

malonic ester
O
!

ester enolate ion
O

Malonic ester enolates are particularly useful in Michael additions because they are weaker bases than normal ester enolates

O C H OR3

O CH R1 CH R2

OR3 O CH CH R3 O R1
!

R3 O

CH C O

R2

ester enolate ion

#,'-unsaturated carbonyl compound

enolate ion

123

Michael Additions
• The enolate ion that is formed from the addition reaction then goes on to deprotonate another molecule of the original ester.
– This forms the neutral addition product and regenerates an ester enolate anion.

•

Thus, this reaction is catalytic in base: only a small amount of base is required (compare to Claisen condensation, which requires one equivalent of base)
– once the original base forms some ester enolate anion, the reaction proceeds without further need of the base.
O R3 O CH H O
O O
!

OR3

C R3 O

C H O OR3

OR3

O OR3 O CH CH R3 O R1
!

another ester enolate anion is formed

O

CH C O

R2
R3 O HC CH R1

CH2 C O

R2

Michael addition product is formed

124

Synthesis With Michael Additions
• Working backwards from the Michael addition product, we can envision two ways of putting the molecule together. O
O